Scientists have genetically modified a human embryo for the first time. Junjiu Huang, a gene-function researcher at Sun Yat-sen University in Guangzhou, China, confirmed that his team has modified the gene responsible for a potentially fatal blood disorder in non-viable embryos — ones that cannot result in a live birth.

They reported their work, which up until now was merely a rumor, in this week’s online journal Protein & Cell.

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The results will no doubt reignite an already highly contentious debate between those who think editing the genome of a human before it’s born could prevent genetic disease and those who believe the unpredictable effects could be devastating to humankind.

Huang’s team used a technique well known in genetic science called CRISPR/Cas9. Essentially, scientists inject an embryo with an enzyme of the same name that can be programmed to bind and splice DNA at a specific gene location. During the process, another molecule is introduced to that location to repair genetic damage.

Until now, no one has reported trying this technique on a human embryo.

The Chinese team tried it on not just one human embryo, but 86. These were obtained from fertility clinics and were unviable because they all contained an extra set of chromosomes after being fertilized by two sperm.

After injecting the enzyme into the embryos, the team waited 48 hours, enough time for the enzyme to splice the gene, for the molecule to replace the missing DNA and for the single-celled embryos to grow to about eight cells each.

Seventy-one embryos survived, and of those, 54 were tested and of those, only 28 showed a successful splice. Even more disconcerting, only a fraction of the 28 embryos contained the replaced molecule.

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Huang admitted that the experiment was not a success — closer to a 100 percent success rate would be needed for such a procedure — and as a result, the team stopped, calling the technique immature.

Even with a 100 percent success rate, however, plenty of scientists warn that this kind of technique would be unethical. Genetic changes to an embryo are passed along to offspring and the effects on future generations are unpredictable.

But Huang is not shelving the experiments. According to Nature, he will consider alternative strategies for tweaking and/or administering the enzymes.

And apparently he isn’t alone. David Cyranoski and Sara Reardon reports in Nature News that at least four groups in China are pursuing gene editing in human embryos.